The present invention relates to a method for modulating the pressure reduction when controlling the wheel slip by means of an anti-lock hydraulic brake system. Electric signals representative of the wheel rotational behavior are generated, reconditioned and electronically processed. For slip control, or upon the occurrence of a tendency to lock, a pressure control valve is actuated pulsewise and thereby pressure fluid out of the wheel brake cylinder of the wheel concerned will be returned to a pressure compensating reservoir. The braking pressure variation is conformed to the wheel rotational behavior by varying the pulse/-pulse-pause ratio of the valve-actuating signals.
Anti-lock hydraulic brake systems for vehicles are known which contain wheel sensors enabling generation of electric signals indicative of the wheel rotational behavior. Inserted into the pressure fluid lines from the braking pressure generator to the wheel brake cylinders are solenoid valve assemblies composed of an inlet valve and an outlet valve. The assemblies serving to keep the braking pressure constant or to reduce it upon an imminent locked condition. In this arrangement, the velocity or the gradient of the braking pressure build-up and reduction is varied by means of pulse-shaped valve-actuating signals, the pulse/-pulse-pause ratio of which is variable between zero and infinity as a function of the wheel motional behavior (German patent specification No. 19 14 765). The pressure against time discharge characteristic curve or the pressure variation in the phase of pressure reduction is not considered or only considered indirectly and thus to an insufficient degree. Due to the known nonlinearity of the pressure/time characteristic curve of a brake system, there occurs a very rapid initial pressure reduction while later the pressure/time characteristic curve extends very flatly. The initially very high pressure-reduction amplitude makes it difficult to exactly meter the pressure reduction and favors the tendency of the wheel suspension to oscillate. Further, driving comfort suffers from the initially very high amplitudes by which the pressure is varying. In particular, on slippery roads the slow pressure reduction in the lower range of the pressure/time characteristic curve proves to be a disadvantage.
Therefore it is an object of the present invention to overcome the disadvantages described and to develop a method for the control of an anti-lock hydraulic brake system which, for regulation of wheel slip, enables attainment of a finer proportioning of the pressure reduction and a better adaptation of the pressure to the various frictional coefficients, road conditions and situations.